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I believe you're overwriting them every time because you're using the same marker id. If you increment the marker id each time, then you'll be adding a new marker each time rather than modifying the existing marker each time. You can control how long each will stay by using the lifetime field or by explicitly publishing a Marker message with the same marker id and the action delete.

If you look at the marker message, the actions ADD and MODIFY have been merged. Really it should be something like SET, where that actually means "set contents for this marker id".